Students try a motor straight on pin 9, compare the 700 mA it asks for with the 20 mA a pin gives, follow the two currents in a transistor circuit, see and fix the switch-off spike with a diode, and sort parts by whether a pin can drive them.
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Students will be able to
I can explain why a pin can't run a motor.
I can describe how a transistor lets a pin switch a motor.
I can say what a flyback diode is for.
Timing
Predict and Try 5 min · Worked example 8 min · Vary 6 min · Tinkercad 12 min · Practise and Make 9 min
Before class
Model: Uno pin 20 mA recommended, 40 mA absolute maximum; motor 150 mA running, 700 mA starting (a small 6 V TT gear motor; check your kit's datasheet). In reality a pin asked for more sags and can be damaged; the board shows the current asked for.
Safety: never wire a motor straight to a pin on real hardware. Unit 4 (this course) is Years 9–10.
Running the lesson
Predict: ask what is different about a motor and an LED, apart from what it does.
Worked example: trace both loops in two colours.
Vary: the spike is invisible on real hardware until something dies; say so.
The hard part (on purpose)
Students reach for the code first. Make 'power before code' the rule of the unit.
When students get stuck
What you will see
A question to ask
The fix
A student rewrites the code.
How much current does the motor need, and how much can the pin give?
Use a transistor or driver.
Answers to the checks
motor on a pin: A small motor is wired from pin 9 to GND. The sketch sets pin 9 HIGH. The motor… It doesn't turn: it needs about 700 mA to start and the pin gives about 20. Asking a pin for far more than 40 mA can damage it.
Try: wire it straight: Switch pin 9 on. Does the motor turn? Compare the current it asks for with what a pin can give.
It asks for about 700 mA; a pin gives about 20. Nothing in the code can change that.
Two currents: The motor needs 700 mA to start. A pin can safely give 20 mA. How many times too much is that? 35 700 ÷ 20 = 35 times what a pin should give.
Find the mistake: Sam's motor won't turn on pin 9. Click the line where Sam's reasoning first goes wrong. line 1 Line 1: an LED needs about 10 mA, a motor hundreds. The pin can't supply it, so no code will fix it: it needs a transistor or driver and its own supply.
Switch off: Switch the motor on, then off. What does the board warn you about?
A motor is a coil: when its current is cut, it makes a voltage spike much bigger than 6 V that can damage the transistor.
Fit the diode: Fit the diode, then switch the motor on and off again.
The diode (across the motor, pointing back to +) gives the spike a safe path round the motor. Motor drivers usually have these built in.
which can a pin drive?: Sort each part: straight from a pin, or through a transistor or driver? An LED with a 220 Ω resistor (about 15 mA) → Straight from a pin (with a resistor if it needs one); A small DC gear motor (150 mA running) → Through a transistor or driver; A piezo buzzer (a few mA) → Straight from a pin (with a resistor if it needs one); A 12 V LED strip (500 mA) → Through a transistor or driver; A servo's signal wire (its power from 5V) → Straight from a pin (with a resistor if it needs one) Anything over about 20 mA needs its own supply, switched by the pin. A servo's signal wire carries almost no current; its power wire goes to a supply.
wire it safely: Change this circuit so the motor runs with the pin safe and the transistor protected.
The pin switches a transistor, the battery drives the motor, and the diode catches the switch-off spike.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: switch it off › Switch off
Open task: any program that does what the step asks.
Vary: switch it off › Fit the diode
Open task: any program that does what the step asks.
wire it safely
Open task: any program that does what the step asks.
Stretch and support
Stretch: A MOSFET (such as the IRLZ44N) can switch several amps with almost no gate current. Why would that suit a bigger motor?
Support: A two-column card: signal (pin, mA) and power (battery, hundreds of mA).
Exit ticket
Why can't you run a motor from an Arduino pin, and what do you use instead?
A pin gives about 20 mA and a motor needs hundreds. The pin switches a transistor or motor driver, which switches the motor's own battery.
What each step is for
motor on a pin: Committing to what a motor does on a pin tests the new move: the code can be perfect and the motor still can't run, because the pin can't supply the current.
Try: wire it straight: Switching the pin on with the motor wired straight to it settles the prediction and shows the numbers: the current asked for and the current a pin can give.
Worked example: the pin only switches: Following both paths, the small control current and the big motor current, models the move. Sam's 'the code must be wrong' is planted after the method.
Vary: switch it off: Switching the motor off without a diode shows the second hazard (the coil's voltage spike); fitting the diode removes it.
Build it in Tinkercad: Tinkercad has a DC motor, an NPN transistor, a diode and a battery pack, so students build the real circuit and see the motor's current on a multimeter.
which can a pin drive?: Sorting parts by the current they need applies the 20 mA rule.
wire it safely: Choosing the wiring and protection from a dangerous starting point brings both hazards together.